The field of the invention is power recovery from a fluid catalytic cracking (FCC) unit. FCC technology, now more than 50 years old, has undergone continuous improvement and remains the predominant source of gasoline production in many refineries. This gasoline, as well as lighter products, is formed as the result of cracking heavier (i.e. higher molecular weight), less valuable hydrocarbon feed stocks such as gas oil.
In its most general form, the FCC process comprises a reactor that is closely coupled with a regenerator, followed by downstream hydrocarbon product separation. Hydrocarbon feed contacts catalyst in the reactor to crack the hydrocarbons down to smaller molecular weight products. During this process, the catalyst tends to accumulate coke thereon, which is burned off in the regenerator. The heat of combustion in the regenerator typically produces flue gas at elevated temperatures of 677° to 788° C. (1250° to 1450° F.) which is an appealing focus of power recovery.
FCC gaseous products exiting the reactor section typically have a temperature ranging between 482° and 649° C. (900° to 1200° F.). The product stream could be an attractive source power recovery but is instead introduced directly into a main fractionation column meaning that no unit operations are interposed on the line between the FCC product outlet and the inlet to the main column. Product cuts from the main column are heat exchanged in a cooler with other streams and pumped back typically into the main column at a tray higher than the pumparound supply tray to cool the contents of the main column. Medium and high pressure steam is typically generated by the heat exchange from the main column pump-arounds. Low pressure steam is typically generated at 241 to 448 kPa (gauge) (35 to 65 psig). Medium pressure steam is typically generated at 1035 kPa (gauge) (150 psig) and high pressure steam is typically generated at approximately 4137 kPa (gauge) (600 psig). For example, a stream from the main column bottom may be circulated through heat exchangers to impart process heating or steam generation. The cooled main column bottoms stream is typically returned above the main column flash feed zone to quench the vapors entering the main column from the FCC reactor. The FCC reactor vapors are cooled from 482° to 649° C. (900° to 1200° F.) to temperatures of approximately 371° C. (700° F.) in the main column flash zone. In this way, the FCC reactor effluent vapors are quenched.
However, steam at greater than these pressures can be used to generate incremental power recovery. Very high pressure (VHP) steam is typically generated at 6200 to 11030 kPa (gauge) (900 to 1600 psig). The FCC reactor effluent vapors are at sufficient temperature to generate steam at the pressure levels required to generate this VHP steam.